Headwaters

n. upper section of a river

Headstream is the origin of water flow that initiates the subject watercourse. It is the start of the river or stream.The source of a river or stream may be a lake, a marsh, a spring, glacier, or a collection of headwaters. For example the source of the River Tees is marshland. The furthest stream is called the headstream. Headwaters are usually small streams that are often cool waters, because of shade and recently melted ice or snow. Also they may be glacial headwaters, waters formed by the melting of glacial ice. The source is normally the farthest point of the river stream from its estuary or its confluence with another river or stream.
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Noun
1. the source of a river; "the headwaters of the Nile"
(hypernym) beginning, origin, root, rootage, source

The upper part of a river system, denoting the upper basin and source streams of a river. (DOI4)

The source, or upper part, of a stream. Often used in discussing water rights related to wilderness or other federal land designations.